An obligation to pay money generally is enforced by an action of assumpsit, and to that extent is referred to a contract, even though it be one existing only by fiction of law. But such obligations when imposed upon the members of a corporation may very very largely. The incorporation may create a chartered partnership the members of which are primary contractors, or it may go no farther than to impose a penalty; or again, it may create a secondary remedy for a debt treated as that of the corporation alone, like the right to attach the corporation's real estate; or the liability may be inseparable from the local procedure; or the law may be so ambiguous as to leave it doubtful whether the liability is matter of remedy, and local, or creates a contract on the part of the members that will go with them wherever they are found (McClaine v. Rankin). In the present case we think that there can be no doubt of the meaning of the California statute. It reads: 'Each stockholder of a corporation is individually and personally liable for such proportion of its debts and liabilities,' etc., as we have stated, and supposes the action against him to be brought 'upon such debt.' Civil Code, 322. This means that by force of the statute, if the corporation incurs a debt within the juris- [232 U.S. 221, 236] diction, the stockholder is a party to it, and joins in the contract in the proportion of his shares.

A fiction of law is stated from Lectric Law Library on the net as follows:
The assumption that a certain thing is true, and which gives to a person or thing a quality which is not natural to it, and consequently establishes, a certain disposition, which, without the fiction, would be repugnant to reason and to truth. It is an order of things which does not exist, but which the law prescribes or authorizes. It differs from presumption because it establishes as true, something which is false; whereas presumption supplies the proof of something true.

The law never feigns what is impossible. Fiction is like art; it imitates nature, but never disfigures it. It aids truth, but it ought never to destroy it. It may well suppose that what was possible, but which does not exist; but it will never feign that what was impossible actually is.

Fictions were invented by the Roman praetors who, not possessing the power to abrogate the law, were nevertheless willing to derogate from it under the pretense of doing equity. Fiction is the resource of weakness which, in order to obtain its object, assumes as a fact what is known to be contrary to truth: when the legislator desires to accomplish his object, he need not feign, he commands. Fictions of law owe their origin to the legislative usurpations of the bench.

It is said that every fiction must be framed according to the rules of law, and that every legal fiction must have equity for its object. To prevent their evil effects, they are not allowed to be carried further than the reasons which introduced them necessarily require.

The law abounds in fictions. That an estate is in abeyance; the doctrine of remitter, by which a party who has been disseised of his freehold and afterwards acquires a defective title, is remitted to his former good title; that one thing done today, is considered as done at a preceding time by the doctrine of relation; that because one thing is proved, another shall be presumed to be true, which is the case in all presumptions; that the heir, executor, and administrator stand by representation in the place of the deceased are all fictions of law.

Now notice that Assumption is the word used to describe how fiction operates. The word presumption is the opposite, see opening paragraph for this sentence "It differs from presumption because it establishes as true, something which is false; whereas presumption supplies the proof of something true."

Now lets use this material from Black's 3rd Edition Law book and a case to wit:
Fiction. Derived from Fictio in Roman Law, a fiction is defined as a false averment on the part of the Plaintiff which the defendant is not allowed to traverse, the object being to give the court jurisdiction. Black's Law Dictionary 3rd Ed. (1969)